That is generally paragon of bad writing right there.
If the game is saying that you are becoming a literal god and only this literal god can kill the enemy by shooting them in the gut with arrows/fireballs/bullets/swords, then it is already going overboard, confusing the mechanics with the story. I would rather something like AoD where you are the joe shmoe who is good at capitalizing a few skills to get what they want. You *can* become a god, yes, but then it comes out of the story as meaningful and not from the mechanics.
What does it have to with writing? You are tasked with killing a 20th level archwizard. In order to achieve that, you need to become 20th level yourself. If every village can then field a squad of 25th level cops in case that you start pickpocketing hobos, why the fuck did they bother you with that archwizard in the first place?
Oh, and protip, whatever you throw against the player, some inventive player will be able to prevail over, which leaves you with "a million rock cops fall, you die" copout. Which is far more despicable and career-ending failure than any number of "essential" NPCs.